The high pressure forms of boron nitride, known as cubic boron nitride (cBN) and wurzitic boron nitride, are surpassed only by diamond in hardness and have a wide variety of uses as machining tools, abrasives, wire dies, wear parts, heat sinks, and the like.
Wurzitic boron nitride, typically formed by shock or explosive techniques, has a hardness equal to cBN and can be substituted or mixed with cBN in most applications. Wurzitic boron nitride, however, is thermodynamically unstable relative to cBN under conditions favorable to sintering and will revert to cBN in the presence of catalyst-solvents.
cBN, in particular, is preferred to diamond in working with ferrous metals because it is chemically more stable than diamond, has a higher temperature threshold for conversion to its hexagonal or graphitic form and is not catalytically degraded by hot ferrous metals, as is diamond. In the applications mentioned above, the primary qualities desired for a polycrystalline cBN compact tool are abrasive wear resistance, thermal stability, high thermal conductivity, impact resistance, and a low coefficient of friction in contact with the workpiece. While cBN itself possess each of these qualities to a significant degree, whether a polycrystalline compact of cBN as a whole possesses them will depend largely on the characteristics of the other materials that will make up the compact, i.e., binder material, catalysts, substrates, and the like, along with processing parameters such as particle surface cleanliness, grain size and the like.
Compacts having a lower concentration of cBN and a higher concentration of a nonmetallic adjuvant material have been favored for such applications. Although it is possible to form a sintered compact of cBN with no adjuvant material under conditions of high pressure and temperature, strongly adherent surface oxides of boron act to inhibit significant intergranular bonding and make it difficult, if not impossible, to obtain an adequate compact strength. Various adjuvant materials are thus incorporated, either to enhance intergranular bonding or to surround the grains with a continuous somewhat less brittle matrix, producing a stronger cBN compact. The adjuvant material may also impart other desirable physical characteristics to the compact such as chemical resistance and impact resistance depending on the particular adjuvant material chosen. Additionally, the use of adjuvant materials helps to reduce the material cost associated with producing the cBN compact due to the decreased amount of cBN crystals required as a starting material.
The adjuvant materials chosen should possess two general sets of qualities; (1) mechanical and chemical properties as close to those of cBN as possible, so as not to deteriorate tool performance, and (2) characteristics enabling manufacture of the compact, such as a melting point at readily obtainable temperatures or good plasticity at such temperatures, limited but not excessive chemical reactivity towards cBN, and most preferably, catalytic-solvent activity for conversion of hexagonal boron nitride to cBN. This latter characteristic will facilitate crystalline growth and intergranular bonding under conditions of pressure and temperature at which cBN is thermodynamically stable.
The use of adjuvant materials as catalyst-solvents for conversion of hexagonal boron nitride to cBN are disclosed in the prior art. U.S. Pat. No. 3,918,219 to Wentorf discloses a method for converting hexagonal boron nitride to cubic boron nitride in the presence of catalyst material. Hexagonal boron nitride (hBN) is the low pressure graphitic powder form of boron nitride. hBN alone, under conditions of elevated temperature and pressure does not form a sintered polycrystalline cBN compact containing the necessary physical properties to be useful as a machining tool. Instead, hBN forms a weakly intergranular bonded cBN structure having a high degree of interstitial voids that tends to exfoliate. However, hBN can be used as a suitable starting material and will yield a desirable compact if a limited amount is combined with cBN crystals and an adjuvant material.
The use of cBN crystals as a starting material is known in the art. U.S. Pat. No. 4,647,546 to Hall discloses a process for making a polycrystalline cBN compact by combining cBN with suitable adjuvant materials. cBN is indispensable for imparting the excellent properties of abrasive were resistance and chipping resistance to the high pressure high temperature sintered compact. Further, cBN crystals act as nucleation sites when combined with hBN and adjuvant materials to facilitate the formation of the polycrystalline cBN structure during sintering.
Aluminum containing materials have certain desirable properties which have led to their use, separately, in prior art compositions. Use of aluminum as an aid in bonding cBN under high pressure, temperature conditions is taught by U.S. Pat. No. 3,944,398 to Bell. Bell teaches the use of a material consisting of a boride, nitride, or silicide refractory substance and a solvent of aluminum, lead, tin, magnesium, lithium, or alloys thereof. The preferred embodiment of Bell, employs silicon nitride as the second refractory substance and aluminum as the solvent. Bell teaches that substantially all of the aluminum reacts with the silicon nitride to form aluminum nitride. The resulting cBN compact displays good thermal stability, enhanced impact resistance and performs well in aggressive cutting operations of hard ferrous alloys. However, the large amount of binder materials used, which are considerably softer than cBN, tended to interfere with intergranular cBN to cBN bonding and adversely affected the abrasive wear resistance of the sintered cBN compact.
Another hard material used in combination with cBN is one selected from a carbide, nitride, or carbonitride of a group IVb, Vb, and VIb transition metal of the periodic table. U.S. Pat. No. 4,334,928 to Hara discloses cBN compacts made with hard materials selected from carbides, nitrides, carbonitrides, borides, and silicides of the group IVb, Vb, and VIb transition metals. Hara also teaches that a catalyst such as aluminum and/or silicon, may be added to the composition in a small amount. The carbide, nitride, or carbonitride containing hard materials are chosen because of their ability to impart to the cBN compact enhanced chemical and impact resistance. The Hara patent neither intends nor achieves substantial direct cBN to cBn intergranular bonding, in part due to the low concentration of cBN. Accordingly, the low volume concentration of cBN and lack of substantial intergranular bonding produced a cBN compact having poor wear resistance in abrasive applications.
U.S. Pat. No. 4,619,698 to Ueda discloses very high pressure sintered compacts of cBN containing at least one metal selected from the group consisting of cobalt and nickel. The use of cobalt as a binder material has been shown to improve the degree of sintering of the CBN compact.
Although the prior art discloses the advantages of making a cBN compact using a variety of adjuvant materials, it does not disclose the process of combining these or other adjuvant materials in the appropriate amount to produce an improved sintered polycrystalline cBN compact. Further, the methods described in the prior art are not the most economically advantageous methods for making the cBN compact because of the excessive material cost associated with using a high proportion of cBN crystals as a starting material.
It is therefore highly desirable to provide a method for making a sintered polycrystalline cBN compact, comprising the use of various adjuvant materials that act to facilitate the conversion of hBN to cBN and enhance the strength and degree of intergranular cBN to cBN bonding of the polycrystalline cBN compact and impart to the sintered cBN compact the level of abrasive wear resistance, impact resistance, thermal conductivity and stability needed to perform as a cutting tool. It is also desirable that the method of making the polycrystalline cBN compact be cost effective in terms of starting material costs.